As semiconductor devices become more and more dense, it has become necessary to use multiple conductive layers in order to successfully manufacture these dense devices while retaining relatively small package sizes. This is particularly true with dynamic random access memories as memory density has increased while the space allowed for each storage capacitor has decreased. Due to the need to save die space while maintaining capacitance, approaches have been taken to save space by insulating the capacitor's electrode (normally polysilicon and referred to hereinafter as cell poly) from a contact forming filler (referred to hereinafter as a contact plug or poly plug).
A conventional method to accomplish isolation of a cell poly from the contact plug requires the use of a contact filler definition pattern and poly etch. The pattern would be applied to the cell poly layer and then dry etched in order to form windows in the cell poly thus allowing for the contact placement. This method has the disadvantage of requiring enough sizing to insure that no connection could be made from the contact plug to the cell poly. Given the current state of lithographic technology, for example, this would require 0.15 .mu. of spacing from each side of the window to the contact filler. This would result in the final sizing of the die to be increased by 0.3 .mu. per DRAM cell location.
The method proposed, herein, uses a penetrating cell plate contact technology to recapture the 0.3 .mu. of space consumed by the old technology. This method is achieved by not patterning the cell poly until the actual contact is being etched. The contact etch must etch, not only down to, but also through the cell poly, (anisotropically or isotropically depending on the application) and finally etching down to the substrate to finish the contact etch. Before the contact plug can be applied to the contact opening, the cell poly must be electrically isolated so that it is not possible for a conduction path to be present between the cell poly and the substrate.
A couple of such methods have been studied for cell poly isolation from a poly plug. One method involves oxidizing the cell poly with a wet oxidation step with the results depicted in FIG. 1. As FIG. 1 shows, the silicon 11 at the bottom of the contact will oxidize much faster than the cell poly 12. This results in having a thick oxide layer 13 to etch out of the bottom of the contact after the oxidation is complete. Using either a selective etch or stopping the etch short of clearing the silicon from oxide will leave behind a thick layer of oxide 13 in the bottom of the contact area, which is an undesirable side effect.
A second method uses a nitride deposition to insulate the cell poly from the poly plug as depicted in FIG. 2. This method results in the same problem as the oxidation method in that now a presence of thick nitride 21 residing in the contact area must be removed.
Unfortunately, both methods listed above increase the defect density problem due to the fact that a contact must be etched twice which doubles the chance of particles, generated from the etch chamber(s), to block the etch.
Another problem associated with double etching of the contacts is that during the second etch no photoresist is present. The presence of photoresist increases the carbon content of the plasma which improves the selectivity of the etch locally to the substrate and to the contact alignment spacers. This lack of local selectivity could cause excessive removal of substrate which is very undesirable if a shallow junction exists in this contact area. The loss in selectivity could also open areas of the poly gate and allow for unwanted contact to the poly plugs.
A new approach that provides cell poly isolation from a poly plug is needed that subjects the contact to a single etch is in fact accomplished by the present invention.